So-called "hard" contact lenses and compositions for fabricating them are well known in the art. The standard contact lens used for many years was fabricated from poly(methyl methacrylate) (PMMA) and achieved widespread use throughout the world, despite the fact that a PMMA lens is essentially impermeable to oxygen. The lack of oxygen at the interface between the contact lens and the cornea will, after even short periods of time, i.e., less than one day, cause discomfort to the wearer because of edema induced by the lack of oxygen at the surface of the cornea. Consequently, PMMA lenses must be removed at least daily and preferably more often to expose the surface of the cornea to ambient atmospheric oxygen and thereafter replaced, a bothersome procedure.
In attempts to remedy the inherent defects in the standard poly(methyl methacrylate) lenses, numerous polymer compositions have been developed which have demonstrated improved oxygen permeability thus permitting the user to wear the contact lenses for a longer period of time. Polymers used to obtain an improvement in oxygen permeability of contact lenses include: soft, crosslinked poly(hydroxyethyl methacrylate); silicone rubber; cellulose acetate butyrate; and methyl methacrylate copolymers with methacrylpolysiloxane and other comonomers.
Soft hydrogel lenses, such as those formed from crosslinked poly(2-hydroxyethylmethacrylate), have a high water content and are in general more comfortable to wear than PMMA lenses in view of their softness and increased oxygen permeability. As a result of their water content, however, they inherently have a limited oxygen permeability and lack the necessary dimensional stability required for a high degree of visual acuity. Moreover, they are fragile and have a tendency to become cloudy because of the ease with which proteinaceous material and other contaminants are deposited thereupon.
Contact lenses formed from cellulose acetate butyrate have been found to have a higher oxygen permeability than those formed from poly(methyl methacrylate) but in turn suffer from a lack of dimensional stability, which results in an undesirable degree of warpage, and lack the hardness required for resistance to scratching. Soft contact lenses prepared from silicone rubber have very high oxygen permeability but poor mechanical properties and cannot be lathe cut. Further as a result of their very low wettability, they tend to attract and hold contaminants and thus prevent the ready removal of toxic products by tears. Such deficiencies have curtailed their commercialization.
Copolymers of methyl methacrylate and poly(fluoroalkyl) methacrylates have been disclosed by Cleaver (U.S. Pat. No. 3,950,315) and copolymers from methyl methacrylate and methacryloxyalkylpolysiloxanes have been described by Gaylord (U.S. Pat. No. 4,120,570), Ratkowski (U.S. Pat. No. 4,535,138) and others. Compared with contact lenses formed from PMMA, contact lenses prepared from the methyl methacrylate copolymers of Gaylord and Ratkowski, supra, offer significantly greater but still limited oxygen permeability. A summary of related art is set forth in Keogh (U.S. Pat. No. 4,259,467) and need not be repeated here.
Ichinohe (U.S. Pat. No. 4,433,125) and Kawamura (U.S. Pat. No. 4,540,761) describe hard contact lens materials made from copolymers of fluoroalkyl methacrylates and methacryloxyalkylsiloxanes which provide a somewhat greater oxygen permeability than earlier methacrylate copolymers described in the art but even the values obtained are still considerably below that desired for a safe, comfortable extended-wear lens.
Rice et al (U.S. Pat. No. 4,440,918) disclose ophthalmic devices prepared from polymers and copolymers derived from telechelic perfluoropolyethers. Certain of these materials have an unusually high oxygen permeability but have the disadvantage of being so soft that they cannot be lathe cut. Copolymers of telechelic perfluoroethers with sufficient methyl methacrylate or other polymerizable materials to provide the hardness required for lathe cutting, however, have greatly reduced oxygen permeability.
Although the hard oxygen permeable contact lenses available up to now represent a marked improvement over standard PMMA lenses, they still lack the degree of oxygen permeability required for safe and comfortable extended wear. Further, the currently available hard oxygen permeable contact lenses have certain disadvantages in comparison with PMMA lenses in terms of mechanical strength, durability, dimensional stability and wettability.